Communication pubs.acs.org/crystal
Control over Alignment and Growth Kinetics of Si Nanowires through Surface Fluctuation of Liquid Precursor Yi-Seul Park,† Da Hee Jung,† and Jin Seok Lee* Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Korea S Supporting Information *
ABSTRACT: Control over alignment and growth kinetics of vertically aligned Si nanowire (v-SiNW) arrays, which were grown using chemical vapor deposition (CVD) via a metal catalyst-assisted vapor−liquid−solid (VLS) mechanism, was demonstrated by introducing a homemade bubbler system containing a SiCl4 solution as the Si precursor. Careful control over the bubbler afforded different amounts of SiCl4 supplied to the reactor. By varying the dipping depth (Dd) and tilting angle (Ta) of the bubbler, the SiCl4 precursor concentration would fluctuate to different degrees. The different SiCl4 concentrations afforded the fine-tuning of v-SiNW array properties like alignment and growth kinetics. The degree of alignment of v-SiNWs could be increased with large amounts of SiCl4, which was caused by slight shallow depth or gentle tilting of the SiCl4 solution in the bubbler due to an increasing degree of fluctuation and fluctuation area. The ability to control alignment and growth kinetics of v-SiNW arrays could be employed in advanced nanoelectronic devices.
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sidewalls. However, it is difficult to realize v-SiNWs using SiH4. On the other hand, the SiCl4 precursor affords the best results with respect to epitaxial growth of SiNWs. In this case, the gaseous hydrochloric acid (HCl), resulting from SiCl 4 decomposition, etches the native oxide layer on the Si surface presenting a clean crystal surface for epitaxial SiNW growth.18,19 In general, control over diameter,20 length,21 density,22 crystallographic orientation,23 and crystallinity24 of NWs enables the regulation of their electrical, optical, and mechanical properties. This is commonly performed by modulating experimental parameters such as temperature, pressure, and gas flow rate.25,26 According to the VLS mechanism, the amount of precursor generally affects the growth kinetics of NW, leading to a difference on both its morphology and alignment.27 Based on the phase-diagram for NW growth,28 these phenomena indicate that the amount of precursor dissolved into the metal catalyst is constant; therefore, it can influence the growth kinetics. In the case of precursor in the gas phase, its amount can be controlled by regulating its flow rate or using mixture gas.29 The amount of precursors in the solid or liquid phases can be adjusted by the temperature of the bubbler, containing a precursor, or the flow amount of carrier gas30 because they can affect the vapor pressure of the precursors. Analogously, we can deduce that the fluctuation of
emiconductor nanowires (NWs) have been known to possess fascinating physical properties that are not observed in their bulk counterparts, based on the quantum confinement derived from their low-dimensional structure.1,2 Of these various NWs, Si nanowires (SiNWs) have proven particularly attractive for use as building blocks in the development of novel nanodevices, including field-effect transistors,3 solar cells,4 optical filters,5 and biological applications.6 Recently, it has even been reported to employ vertically aligned SiNWs (v-SiNWs) for electrical and optical devices. One promising application may be in fabrication of array devices such as vertical field-effect transistors, which could offer higher transistor densities, novel three-dimensional logic architectures,7 and filter-free color imaging.8 However, a prerequisite for the use of v-SiNWs in these aforementioned applications is that their electrical properties are readily understood and controllable, which in turn is strongly dependent on the structural parameters and growth kinetics of the SiNWs.9−11 Conventional vapor-phase fabrication methods for SiNWs include thermal evaporation,12 laser ablation,13 molecular beam epitaxy,14 and chemical vapor deposition (CVD).15 Among them, the CVD method combined with the metal catalystassisted vapor−liquid−solid (VLS) mechanism is the most widely used approach for growth of single crystal SiNWs.16 In the case of single-crystalline SiNW synthesis by CVD, silane (SiH4) and silicon tetrachloride (SiCl4) are the most common precursor when assisted by a Au catalyst.17 The formal approach enables the decomposition of SiH4 at a relatively low temperature, which favors film deposition on SiNW © XXXX American Chemical Society
Received: July 27, 2016 Revised: October 22, 2016 Published: November 2, 2016 A
DOI: 10.1021/acs.cgd.6b01120 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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shows Si peaks at 28° and 47°, corresponding to the (111) and (220) planes, respectively. All samples had Au(111) and Au(200) reflections at 38° and 44°, from the Au catalyst alloy at the top of the SiNW.16 The XRD patterns show a major Si(111) peak when there are only v-SiNWs due to epitaxial growth of the SiNWs. In order to compare the degree of alignment of v-SiNW arrays, the ratio of the Si(111) to Si(220) peak intensities (I(111)/I(220)) was investigated.33 The values of I(111)/I(220) are 9.31, 1.17, and 1.06 for Dd of 0.5, 1.0, and 1.5 cm, respectively. The data suggest that increasing Dd leads to a decrease in the degree of alignment of SiNWs, consistent with the results of tilted view SEM images (Figures 1a−c). In addition, the diameter and length of v-SiNWs decrease with increasing Dd. The diameters of the v-SiNWs were 204, 163, and 108 nm with corresponding lengths of 5.35, 4.45, and 2.97 μm for Dd values of 0.5, 1.0, and 1.5 cm, respectively (Figures 1e and S2). The amount of SiCl4 introduced using the bubbler system associated with Dd was measured by recording the time to reduce the same amount of SiCl4 solution in the reservoir (Figure S1) with varying Dd to identify the amount of SiCl4 supply into the reactor. The amount decreased with increasing Dd because increasing the Dd induced the reduction of fluctuation at the liquid surface of SiCl4 solution by far away from the bubbling location, leading to a short supply of Si precursors (Figure 1f). The diameter and length increased by reducing Dd, which indicated that the reaction kinetics for SiNW growth were fast. In particular, the increasing diameter was caused by lateral growth due to fast reaction kinetics.34 When the Dd values were changed from 0.5 to 1.5 cm, the diameter of SiNWs reduced by half. If the diameter is small, it is difficult to match the lattice between SiNWs and Si(111) wafers for epitaxial growth, leading to reduction of uniformity in SiNW arrays.34 Furthermore, Dd values below 0.5 cm were examined to parse its effect on alignment of v-SiNW arrays. When SiNWs were synthesized with Dd of 0 cm, v-SiNWs were populated as shown in Figure S3. In this case, bubbling of the SiCl4 solution did not occur, which led to fluctuation only on the liquid surface of the SiCl4 solution. As such, Si precursor supply into the reactor during SiNW growth was insufficient, leading to poor alignment of v-SiNW arrays. The as-grown v-SiNWs were also inhomogeneous, which led to different substrate colors depending on the viewing location, as shown in the inset of Figure S3b. According to these results, Dd of at least 0.5 cm is required to improve the alignment. Figure 2a−c shows 45° tilted view SEM images of v-SiNW arrays synthesized at 860 °C for 5 min on a Si(111) wafer covered with a 5 nm Au film and Ta values of (a) 10, (b) 30, and (c) 50°. The Dd values were fixed at 0 cm, which means that there are only surface fluctuations of liquid precursor as the supply method of SiCl4 into the reactor. The alignment of vSiNW arrays was improved by decreasing Ta, which was identified by XRD analysis. The Si(111) to Si(220) peak intensities (I(111)/I(220)) were 178.04, 92.19, and 4.09 for Ta values of 10, 30, and 50°, respectively (Figure 2d). To understand the impacts of Ta on v-SiNW arrays, the diameter and length of the SiNWs were measured and plotted as a function of Ta in Figure 2e. By increasing Ta, the SiNWs were long and thin. This effect manifested in the aspect ratios of 24.35, 48.05, and 71.93 for Ta values of 10, 30, and 50°, respectively (Figure 2e). The amount of SiCl4 introduced by the fluctuation associated with Ta was measured as shown in Figure 2f. The amount decreased with increasing Ta due to a small surface area for fluctuation at the liquid surface, leading to
liquid precursor in the bubbler also influences the transported amount of precursors into metal catalyst, resulting in different morphology and alignment of NWs. In this study, we focused on the correlation between fluctuation of liquid precursor in the bubbler and alignment of SiNW arrays, which were grown using CVD via a metal catalyst-assisted VLS mechanism. Based on the fact that the amount of Si precursor introduced into the reactor affects the growth kinetics of SiNWs, we first controlled the amount of Si precursor by regulating the fluctuation of liquid precursor in the bubbler containing a SiCl4 solution as the Si precursor. Varying the dipping depth (Dd) and tilting angle (Ta) of the bubbler can change the degree of bubbling in the SiCl4 solution or fluctuation at the surface of liquid precursor, leading to difference in the amounts of Si precursor supplied to the reactor. Remarkably, the vertical alignment of SiNWs was improved by reducing the Dd and Ta, which caused the large supply amount of Si precursors into the reactor, with small aspect ratio. Furthermore, we also investigated the effect of fluctuation of liquid precursor by directly controlling the carrier gas into the bubbler. The v-SiNW arrays were synthesized via the VLS mechanism using a 5 nm thick Au film as a metal catalyst (Scheme 1, Scheme 1. Schematic Illustration of (a) a Furnace Used for the Synthesis of v-SiNW Arrays and (b) the Homemade Bubbler System To Control Dipping Depth (Dd) and Tilting Angle (Ta) of SiCl4 as a Si Precursor for SiNW Growth
Materials and Methods and Figure S1 in detail in Supporting Information).31,32 Upon reaching the reaction temperature, liquid-phase Au nanoparticles (AuNPs) were randomly formed from Au thin film, and then vapor-phase Si precursors were introduced into the liquid-phase AuNPs. The single crystal of Si was precipitated by reaching supersaturation in the Au−Si alloys, which had the irregular diameter, resulting in lack of uniformity in the growth of SiNWs.33 Our bubbler system consists of reservoir for SiCl4 solution as Si precursors in the upper part and bubbler for control of the Dd and Ta in the bottom part as shown in Figure S1. The bubbler was connected to the reservoir of SiCl4 solution by flexible tube, resulting in free movement of bubbler for controllable Dd and Ta. During the reaction, the carrier gas flowed into the bubbler through the input and the vaporized Si precursors caused by bubbling went out the bubbler through the output. In order to verify the effect of Dd, the syntheses were conducted at 860 °C for 5 min on a Si(111) wafer with Dd values of 0.5, 1.0, and 1.5 cm (Figures 1a−c). As Dd increased, the degree of alignment of v-SiNW was decreased with a misalignment of nonuniform diameter and length. These results were analyzed by XRD (Figure 1d), which B
DOI: 10.1021/acs.cgd.6b01120 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 1. SEM images (45° tilt-view) of v-SiNW arrays after VLS growth on a Si(111) substrate at different Dd values of (a) 0.5, (b) 1.0, and (c) 1.5 cm. Scale bars are 2 μm. Insets show the bubbler with different Dd. (d) XRD patterns showing the alignment of SiNW arrays as a function of Dd. (e) Plot of the diameter and length of SiNWs as a function of Dd. (f) Plot of SiCl4 flow rate as a function of Dd.
Figure 2. SEM images (45° tilt-view) of v-SiNW arrays after VLS growth on a Si(111) substrate at different bubbler Ta values of (a) 10, (b) 30, and (c) 50°. Scale bars are 2 μm. Insets show the bubbler with different Ta. (d) XRD patterns showing the alignment of SiNW arrays as a function of Ta. (e) Plot of the diameter and length of SiNWs as a function of Ta. (f) Plot of SiCl4 flow rate as a function of Ta.
reactor. This in turn leads to poor alignment and an increase in the aspect ratio of the v-SiNW arrays. The effect of the Si precursor amount on SiNW growth36 was further probed by changing the carrier gas. The carrier gas can directly modulate the amount of SiCl4 since it is passed through the SiCl4 solution in the bubbler. The following experimental parameters were used: Dd = 1.5 cm and Ta = 50°. Previously, these parameters resulted in v-SiNW arrays with poor alignment and high aspect
a short supply of Si precursors (Figure 2f). Therefore, when the Ta value decreased, the increased amount of SiCl4 supply into the reactor leads to fast reaction kinetics,35 and the change in aspect ratio with respect to Ta is considerable. These data demonstrate the ability to control the reaction kinetics by regulating the SiCl4 precursor concentration with this specific bubbler system. In particular, an increased Dd and Ta resulted in a corresponding decrease in SiCl4 supplied to the C
DOI: 10.1021/acs.cgd.6b01120 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. SEM images (45° tilt-view) of v-SiNW arrays at a fixed dipping depth (1.5 cm) using (a) small amount of carrier gas (100 sccm) and (b) large amount of carrier gas (140 sccm); at a fixed angle (50°) using (d) 100 sccm and (e) 140 sccm as the carrier gas flow rates. Scale bars are 2 μm. Insets show the cross-sectional SEM images of the SiNWs. Scale bars of insets are 5 μm. Plot of the diameter and length of SiNWs as a function of fixed (c) Dd and (f) Ta with small (100 sccm) and large (140 sccm) amounts of carrier gas.
ratios (Figures 1c and 2c). As such, this platform affords clear identification of the dramatic variation in alignment and aspect ratio by varying the carrier gas. Figure 3 shows the tilted view SEM images collected from v-SiNW arrays synthesized on a Si(111) wafer covered with a 5 nm Au film, with either Dd = 1.5 cm or Ta = 50°. Both small (100 sccm) and large (140 sccm) amounts of carrier gas were used while the total flow rate was identical in each experiment (Figure 3). The small amount of carrier gas with a flow rate of 100 sccm (Figure 3a,d) resulted in poor vertical alignment. However, a remarkable enhancement of the vertical alignment was observed when carrier gases with a flow rate of 140 sccm were used (Figure 3b,e). Furthermore, the alignment of v-SiNW arrays synthesized with either Dd = 0.5 cm or Ta = 10° was also investigated by varying the amount of carrier gas (Figure S4). These results were identical to those in Figure 3. Increasing the amount of carrier gas resulted in better alignment, which corresponds with small Dd and Ta. Figure 3c,f plots the diameter and length of SiNWs as a function of amount of carrier gas with Dd = 1.5 cm and Ta = 50°, respectively. The diameters of v-SiNWs were (a) 186, (b) 469, (d) 131, and (e) 180 nm with corresponding lengths of (a) 4.66, (b) 5.86, (c) 6.77, and (e) 4.85 μm, respectively. These data indicate that the amount of carrier gas can affect the diameter by increasing the lateral growth of SiNWs with large amounts of SiCl4. The relationship between the Si precursor amounts and the (a,b) Dd and (c,d) Ta values is shown in Figure 4. There are two types of Si precursor supply methods in the reactor using bubbler system. One is the bubbling, which occurred at the liquid/gas interface in the SiCl4 solution by introducing carrier gas through input in the bubbler. The other is the fluctuations of liquid precursor at the surface of SiCl4 solution ascribed to carrier gas flow on and into the SiCl4 solution. When the amount of carrier gas was fixed, the amount of Si precursors induced by bubbling in the SiCl4 solution is under the same for both (a) Dd = 1.5 cm and (b) Dd = 0.5 cm. However, the degree of fluctuation on the surface of the SiCl4 solution is
Figure 4. Illustration depicting the fluctuation affected by (a,b) Dd and (c,d) Ta of bubbler, which contains SiCl4 as a Si precursor.
different from Dd values. If the shallow depth is low, the distance between the location of the bubbling and the surface fluctuations of liquid precursor as the supply method of SiCl4 into the reactor is closer than deep dipping. As a result, the degree of surface fluctuation was increased. Bubbling precursors in the solution and the fluctuation at the surface of the solution resulted in a net increase in the Si precursor amount introduced into the reactor with slight dipping depths. Likewise, adjusting Ta value caused the fluctuation at the surface of the SiCl4 solution to be different when comparing Ta values of (c) 50 and (d) 10°. If the tilting angle becomes too small, the surface area of solution broadens; hence, more bulky surface is fluctuated gently in the bubbler. Therefore, the total amount of Si precursors increases with descending tilting angle. In summary, the effects of SiCl4 fluctuation in a homemade bubbler on the growth of v-SiNW arrays was demonstrated. Specifically, varying the bubbler dipping depth and tilting angle D
DOI: 10.1021/acs.cgd.6b01120 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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(7) Goldberger, J.; Hochbaum, A. I.; Fan, R.; Yang, P. Nano Lett. 2006, 6, 973−977. (8) Park, H.; Dan, Y.; Seo, K.; Yu, Y. J.; Duane, P. K.; Wober, M.; Crozier, K. B. Nano Lett. 2014, 14, 1804−1809. (9) Cui, Y.; Lauhon, L. J.; Gudiksen, M. S.; Wang, J.; Lieber, C. M. Appl. Phys. Lett. 2001, 78, 2214−2216. (10) Huang, Z.; Fang, H.; Zhu, J. Adv. Mater. 2007, 19, 744−748. (11) Bucaro, M. A.; Vasquez, Y.; Hatton, B. D.; Aizenberg, J. ACS Nano 2012, 6, 6222−6230. (12) Pan, Z. W.; Dai, Z. R.; Xu, L.; Lee, S. T.; Wang, Z. L. J. Phys. Chem. B 2001, 105, 2507−2514. (13) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208−211. (14) Fuhrmann, B.; Leipner, H. S.; Hŏche, H.-R. Nano Lett. 2005, 5, 2524−2527. (15) Yuan, G. L.; Zhao, H.; Liu, X.; Hasanali, Z. S.; Zou, Y.; Levine, A.; Wang, D. Angew. Chem. 2009, 121, 9860−9864. (16) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89−90. (17) Schmidt, V.; Wittemann, J. V.; Gösele, U. Chem. Rev. 2010, 110, 361−388. (18) Hochbaum, A. I.; Fan, R.; He, R.; Yang, P. Nano Lett. 2005, 5, 457−460. (19) Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Lewis, N. S.; Atwater, H. A. Appl. Phys. Lett. 2007, 91, 103110. (20) Park, Y.-S.; Yoon, S. Y.; Park, J. S.; Lee, J. S. NPG Asia Mater. 2016, 8, e249. (21) Morin, C.; Kohen, D.; Tileli, V.; Fauchernad, P.; Levis, M.; Brioude, A.; Salem, B.; Baron, T.; Perraud, S. J. Cryst. Growth 2011, 321, 151−156. (22) Park, Y.-S.; Lee, J. S. Chem. - Asian J. 2016, 11, 1878−1882. (23) Schmidt, V.; Senz, S.; Gölsele, U. Nano Lett. 2005, 5, 931−935. (24) Anttu, N.; Lehmann, S.; Storm, K.; Dick, K. A.; Samuelson, L.; Wu, P. M.; Pistol, M.-E. Nano Lett. 2014, 14, 5650−5655. (25) Krylyuk, S.; Davydov, A. V.; Levin, I. ACS Nano 2011, 5, 656− 664. (26) Tian, B.; Xie, P.; Kempa, T. J.; Bell, D. C.; Lieber, C. M. Nat. Nanotechnol. 2009, 4, 824−829. (27) Park, Y.-S.; Lee, J. S. J. Nanopart. Res. 2014, 16, 2226. (28) Wang, Y.; Schmidt, V.; Senz, S.; Gösele, U. Nat. Nanotechnol. 2006, 1, 186−189. (29) Ho, T.-W.; Hong, C.-N. J. Nanomater. 2012, 2012, 274618. (30) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 3298−3302. (31) Ge, S.; Jiang, K.; Lu, X.; Chen, Y.; Wang, R.; Fan, S. Adv. Mater. 2005, 17, 56−61. (32) Lee, K.-Y.; Shim, S.; Kim, I.-S.; Oh, H.; Kim, S.; Ahn, J.-P.; Park, S.-H.; Rhim, H.; Choi, H.-J. Nanoscale Res. Lett. 2010, 5, 410−415. (33) Park, Y.-S.; Jung, D. H.; Kim, H. J.; Lee, J. S. Langmuir 2015, 31, 4290−4298. (34) Krylyuk, S.; Davydov, A. V.; Levin, I. ACS Nano 2011, 1, 656− 664. (35) Jeong, H.; Park, T. E.; Seong, H. K.; Kim, M.; Kim, U.; Choi, H. J. Chem. Phys. Lett. 2009, 467, 331−334. (36) Chong, S. K.; Dee, C. F.; Yahya, N.; Rahman, S. A. J. Nanopart. Res. 2013, 15, 1571.
affected the total amount of precursors introduced into the reactor, which in turn lead to different growth kinetics. Without manipulating the amount of carrier gas directly, it is possible to control the amount of Si precursors by bubbling and fluctuation of the SiCl4 solution. The alignment of the v-SiNW arrays improved and the diameter of SiNWs increased with shallow dipping depths due to fast growth kinetics by large supply of Si precursors from synergy effect of bubbling and fluctuation. When the bubbler was tilted to small angles, the fluctuation area increased, which similarly supplied larger amounts of Si precursors. Large SiCl4 supplies can be obtained with both shallow dipping and small tilting angles. This phenomenon was confirmed directly controlling the amount of carrier gas. This new synthetic method affords control over alignment and growth kinetics of v-SiNW arrays. As such, it could be a powerful technique for the manipulation of SiNW arrays used as building blocks in electronic and optoelectronic devices.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01120. Photograph of a homemade bubbler system, crosssectional SEM image, additional SEM images (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
Y.-S.P. and D.H.J. contributed equally to this work. Y.-S.P., D.H.J., and J.S.L conceived and designed the experiments. Y.S.P. and D.H.J. synthesized Si nanowires using a CVD method. D.H.J. performed SEM measurements, and Y.-S.P. carried out XRD measurements. Y.-S.P., D.H.J., and J.S.L. wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Nano·Material Technology Development Program (2012M3A7B4034986) funded by the National Research Foundation and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2012-0009562). Additionally, it was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1A2A2A01005556)
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REFERENCES
(1) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddard, W. A., III; Heath, J. R. Nature 2008, 10, 168−171. (2) Zuev, Y. M.; Lee, J. S.; Galloy, C.; Park, H.; Kim, P. Nano Lett. 2010, 10, 3037−3040. (3) Stern, E.; Wagner, R.; Sigworth, F. J.; Breaker, R.; Fahmy, T. M.; Reed, M. A. Nano Lett. 2007, 7, 3405−3409. (4) Garnett, E.; Yang, P. Nano Lett. 2010, 10, 1082−1087. (5) Seo, K.; Wober, M.; Steinvurzel, P.; Schonbrun, E.; Dan, Y.; Ellenbogen, T.; Crozier, K. B. Nano Lett. 2011, 11, 1851−1856. (6) Kang, K.; Park, Y.-S.; Park, M.; Jang, M. J.; Kim, S.-M.; Lee, J.; Chio, J. Y.; Jung, D. H.; Chang, Y.-T.; Yoon, M.-H.; Lee, J. S.; Nam, Y.; Choi, I. S. Nano Lett. 2016, 16, 675−680. E
DOI: 10.1021/acs.cgd.6b01120 Cryst. Growth Des. XXXX, XXX, XXX−XXX
